1. Field of the Invention
This invention relates, in general, to printers of the dot matrix type and particularly to an improved print hammer mechanism.
2. Description of the Prior Art
Dot matrix printers utilizing hammer banks having hammers retracted by a permanent magnet and selectively released by electromagnets so as to impact an ink ribbon against a platen supported print paper by means of an affixed stylus for printing is widely known in the art. Such printers are shown by R. A. Kleist et al in U.S. Pat. No. 4,033,255 and U.S. Pat. Nos. 3,941,051; 4,044,668, and 4,233,894 issued to Barrus et al. While contributing greatly to the art, the speed of operation of such hammer mechanisms is substantially impaired in utilizing magnetic flexure elements to form a portion of the magnetic flux circuitry with a substantial loss of flux density for hammer elements. A magnetic ribbon shield, parallel with and separated from the hammer flexure by an air gap, has been improvised in attempting to correct this problem. Such a system has two air gaps--one between the flexure and electromagnetic pole and a second between ribbon shield and flexure. The second gap becomes large and is a source of high reluctance in conjunction with the long length of flux path. An additional problem affecting printers' speed in the current state of the art is flexure design. Existing flexures are substantially rectangular in shape and are of comparatively low natural frequency. Additional drawbacks common to hammer mechanisms in the art, which have their stylus at the center of percussion, are offset positioning of the longitudinal axes of the stylus and the pole of the electromagnet. Where the center of percussion is not near the point where the hammer strikes against the electromagnetic pole piece, a minute rubbing action between the meeting surfaces results in high wear. It is therefore desirable to increase the flux density of the hammer element by shortening the flux path and by reducing reluctance. It is also desirable to increase the natural frequency of the flexure elements for greater printer speed; to position the stylus at the center of mass of the hammer's head; and to position the stylus at the point where the hammer strikes the pole piece of the electromagnet.